It's really going to depend on group composition.
Gun H(e)avan 3 is a good one for random guns, many of the are crap yes, great weapons for tossing over to gangers and such.
Run & Gun is a solid all round option. Weapons, armor and martial arts are useful to all archetypes.
Run Faster... it has a lot of cool stuff, but it also pushes the boundaries and complexity. So it may not be a great option if there's a lot of new players.
Street Grimoire is a good option if you've got Awakaned characters. Vastly expands on Traditions as well as adding in a ton of new sells and adept powers, as well as a few new spirit types.
Shadow Spells once again expands options for traditions, spells and adept powers.
Lockdown is a bit of an odd one, it has some new stuff like Cyberware Suits, but it's a bit of an edge case.
Data Trails is good if you've got a hacker int he team, especially a decker. If not, well there is some new electronics and software but most of it is focused around new things deckers and technomancers can do.
Chrome Flesh is pretty much a must have if you've got heavily augmented characters, not only does it contain a pile of new cyberware and bioware, it also adds genetech and nanoware. On top of all that there's new qualities that are great for augmentation focused characters as well as a greatly expanded drugs selection.